1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with a means for generating a reverberation effect.
2. Description of the Prior Art
Even a casual listener can detect the difference between musical instruments played in a concert hall and the same instruments played in the home. One of the principal differences in the sound is caused by the acoustics of the playing chambers. Concert halls usually have a construction which provides a long reverberation time produced by a multitude of echoes from partially reflective surfaces. In the home the presence of carpets, drapes, and "acoustical" ceilings quickly attenuates such echoes and thereby produces a rather "dry" and lifeless characteristic for musical sounds.
It has long been recognized that electronic keyboard instruments such as an organ, provide a more "realistic" sound by incorporating reverberation devices which are intended to simulate the reverberation property of large auditoriums. The most commonly used device to create reverberation effects is a metal spring configured to act as an acoustic delay line. The electrical musical signal is converted to a mechanical motion in the spring by means of a transducer similar in function to a speaker driver. A second transducer located at the other end of the spring converts the mechanical vibrations into electrical signals. A portion of the delayed output signal is added to the input signal thereby producing a feedback echo path which produces the desired multiplicity of echoes for the original signal.
The mechanical spring reverberation device is popular because of its low cost. The sound produced by such devices has a mechanical-like tone characteristic which is only remotely similar to the acoustic reverberation quality of a large auditorium.
Along with the current emphasis on digital techniques for musical systems it is obvious that digital techniques would be applied to produce reverberation effects. Unlike the mechanical spring delay line with its transducers producing nonlinear distortion digital storage and signal delay can be accomplished with no distortion by a variety of digital memory devices. A survey of the state of the art in digital reverberation is found in the technical article: Moorer, J. M., "About This Reverberation Business." Computer Music Journal, Vol. 3, June 1979, pp. 13-28.
Most digital reverberation devices are based on the use of unit reverberators which are either configured as all-pass digital filters or as comb filters. Each of the unit reverberators produces a signal delay and signal attenuation. The output from a number of unit reverberators is summed to provide a reverberation effect produced by a multiplicity of echoes.
A second digital mechanization approach is to simply imitate the mechanical spring acoustic delay by means of digital storage devices such as shift registers or addressable memories such as RAM (random access memory).
In U.S. Pat. No. 4,194,426 entitled "Echo Effect For An Electronic Musical Instrument," a system is described in which a musical tone is repeated at a controlled repetitive rate but with decreasing peak amplitude when a key is released from the musical instrument's keyboard. This echo effect is obtained by generating a repetitively decaying amplitude modulation function and does not produce the tonal effect associated with the reverberation produced by multiple reflections in an acoustic chamber such as an auditorium.